Drive train systems of a vehicle are known in principle, which include an electric machine and an internal combustion engine so as to develop an hybrid drive, a separating clutch being provided between the electric machine and the internal combustion engine, so as to separate from each other or connect to each other the electric machine and the internal combustion engine during operation, and the drive train system having an automatic transmission having an hydraulic torque converter and a torque-converter lockup clutch after the electric machine. During the operation using the electric machine, in this case, the internal combustion engine, which has no dedicated starter, is able to be started by the electric machine. The abovementioned configuration permits the electrical driving using the drive by the electric machine and opened separating clutch. At increased power demand or declining battery capacity, the internal combustion engine has to be started so that the storage battery may be loaded and/or the electric machine may be supported. This starting takes place by coupling the internal combustion engine into the drive train, with the electric machine running, in such a way that the electric machine drives the internal combustion engine until the latter runs under its own power. This starting of the internal combustion engine by the electric machine must take place without causing interference in the drive train, especially without a torque change or pulse change noticeable by the driver. The separating clutch between the internal combustion engine, that is to be started, and the running electric machine, that is functioning as starter, is enclosed in such a way, in this instance, that it grips using a specific slip torque. In that way, the internal combustion engine is accelerated until it has approximately reached the same rotary speed as the electric machine. The additional torque required for this has to be provided by electric machine. This is possible relatively easily if the electric machine has sufficient power reserves and torque reserves and the information on the clutch torque is reproducible very accurately and reliably. For this reason, only incomplete operating guidance is possible when the electric machine is only driven with the additional torque for the start of the internal combustion engine. Data concerning the clutch torque are inaccurate with respect to magnitude and also with respect to time characteristic (slipping of the clutch), and may vary greatly over one operating cycle. In order to correct such sources of error, the rotary speed of the electric machine is additionally influenced during the starting of the internal combustion engine. In order to do this, at the beginning of the starting process of the internal combustion engine, the torque-converter lockup clutch of the torque converter is opened. This gives rise to a slip at the torque converter, the desired, unchanged speed of the following drive train having to be held in reserve by a slip speed. The slip, in this context, is a function of the drive torque that is to be transmitted. But then, if the clutch torque, precontrolled to the electric machine, that is, the clutch torque (that is expected and on which the operating guidance is based) and the actual clutch torque present at the separating clutch are clearly different, there is thus a nonnegligible error with regard to expectations and actual conditions, the rotary speed regulation of the electric machine is not able to correct this error sufficiently rapidly, since the processes during the coupling in of the separating clutch and the start of the internal combustion engine run very rapidly and dynamically, not even if, as in the normal case, the respective rotary speeds of the electric machine are very reliable and easily measurable and controllable.